1. Field of the Invention
This invention relates to the fabrication of a mask for use in the optical lithography of microelectronic circuits. More particularly, it relates to the design of such a mask that utilizes outrigger-type phase shifting and to a simplified process for fabricating said design.
2. Description of the Related Art
The complex patterns that ultimately form the structures of microelectronic circuits must first be transferred to a semiconductor substrate by the process of lithography. Optical lithography is one type of lithographic process, in which the pattern is imaged on the plane of the substrate by passing light through a mask (the photomask) onto the substrate. The details of the pattern are formed by openings in the photomask which are transparent to electromagnetic radiation of some frequency.
The mask itself consists of an optically transparent substrate, typically quartz, which is coated with an optically opaque material, typically chromium. Openings are formed in the opaque material, whose shapes correspond to the microelectronic structures and circuit elements to be fabricated on the semiconductor substrate. Light passing through these openings then falls on a photo-sensitive resist medium that covers the semiconductor substrate, developing it and allowing the delineation of the structures and circuit elements on the substrate.
As the size and spacings of microelectronics structures and circuit elements diminish, so must the corresponding openings in the photomask. This, in turn, causes the transmitted light to form diffraction patterns on the photo-sensitive resist medium, which limits the resolution of the image produced. Several techniques have been applied to improve that resolution, most notably the phase shifting of some or all of the transmitted light along the edges of the photomask openings. Phase shifting an electromagnetic wave means shifting its sinusoidal shape relative to that of an identical reference wave. This can be done by passing the wave through a region of different optical path length than that traversed by the reference wave. A shift of 180 degrees (.pi. radians) will cause the peak of the shifted wave to overlay the trough of the reference wave. If such a shifted wave combines with an unshifted wave, at a common image plane, their fields add to zero and the resulting light intensity is zero. It is also possible to attenuate the shifted wave by passing it through an optically absorbing region. If an attenuated, shifted wave is combined with an unattenuated, unshifted wave, only partial cancellation will result.
Significant cancellation of unwanted diffraction lobes at the edges of small mask openings has been achieved by superimposing phase shifted light on the directly transmitted light in such a manner as to cause the phase shifted light to overlay and combine with the diffraction lobes. There are several approaches to achieve this goal, two in particular being rim-type phase shifting and outrigger-type phase shifting. In rim-type phase shifting, a layer of phase shifting material is formed as a narrow band around the inner edge of the mask opening. In outrigger-type phase shifting, a narrow, open annular channel surrounds the mask opening, separated from it by the opaque material that defines the central opening.
Although the efficacy of rim-type and outrigger-type phase shift masks for improving image resolution is accepted in the present art, technical complexities and the cost of such masks has led to a variety of new approaches for fabricating them. In this regard we note the work of Tzu (U.S. Pat. No. 5,853,923) who teaches a method of forming a rim-type attenuating phase shifting mask which requires only a single resist layer and resist developing step. We note also the work of Tzu et al (U.S. Pat. No. 5,888,678) who teach a method for forming a mask that combines a rim-type attenuating phase shift pattern along with a binary mask pattern (not a phase shift type) on the same mask blank. The work of Moon et al (U.S. Pat. No. 5,853,921) teaches a method of more efficiently forming a radiation blocking layer on an alternating phase shift mask by using two different radiation dose levels to develop the resist layer defining the mask pattern. Finally, the work of Lee (U.S. Pat. No. 5,856,049) teaches a method for combining a rim-type phase shift mask portion incorporating two phase shift layers, along with an outrigger-type mask portion, on the same mask.
The present invention teaches a method for significantly reducing the number of steps required in fabricating an outrigger-type phase shift mask. In this respect it differs in its objectives and methods from all the inventions cited above. The method for forming a phase shifting outrigger-type mask according to the present art requires approximately thirteen sequential processing steps. These include two separate depositions of electron-beam sensitive resist layers, with a further deposition of an optically opaque layer on the second resist layer, through which an electron-beam must first pass so as to expose the resist layer beneath. Not only is such a multiplicity of steps inherently costly and time consuming, but the sequential layering of resist layers described above and the subsequent E-beam exposure through a covering opaque layer, requires extremely precise alignment to produce the required pattern definition.